When can you go to Avalon?

Young Wizard, seeing as this is your first post, we'll let this one slide but please avail yourself of the News! link at the top of the page - one click would have given you the answer you seek:

Young Wizards who are a minimum of level 70 and have completed the quest "Through Glass, Darkly" need to speak to Merle Ambrose who will send you off on your adventures with a quest named "This Little Light of Mine".